The gHacks blog details a registry hack that can be made in Windows Vista systems that sets a default folder size for opening a new Explorer window. Seems like something that would be baked into the folder or view settings, but Vista users can tell you that you can get quite random sizes from opening a new window to dig through files or check photos. The hack requires deleting a few registry keys and tweaking another, so definitely make a backup of your registry (File, Export) before proceeding.